The significant burden that results from dengue virus infection combined with the absence of effective vaccines or drugs makes the development of novel therapeutics a high priority. The goal of our research is to identify and optimize compounds that target essential host factors to further our understanding of host-virus interactions and to validate host targets and lead compounds for further development as potential new classes of anti-viral agents. Our strategy is to target essential host factors by developing selective, covalent cysteine-directed inhibitors. The success of covalent inhibitors has been amply demonstrated by the thirtynine FDA-approved drugs that are highly effective in humans. We recently discovered a substituted quinolone, QL-XII-47, that is a potent inhibitor of the four dengue serotypes (EC90 400nM) at concentrations that are non-cytotoxic. Preliminary data indicate that QL-XII-47 acts via a host factor to inhibit DENV at a point late in viral entry. QL-XII- 47 has exhibited inhibitory activity against a number of other viruses within and beyond the Flavivirus family. We will utilize a combination of medicinal chemistry, chemical biology, chemical proteomics, and virology (1) to understand the structural features required to achieve antiviral activity and to obtain analogs with suitable pharmacological properties to enable in vivo experiments; (2) to examine the target and mechanism of action responsible for the potent antiviral activity of QL-XII-47 and related compounds in vitro; and (3) to examine the spectrum of QL-XII-47s antiviral activities against additional Biodefense Viruses. The goal of this proposal is to identify covalent inhibitors of essential host-factor targets that exhibit broad antiviral activity in vitro and that are efficacious in a murine model of dengue virus infection. Further development of this agent and elucidation of its molecular target(s) may provide the first host-directed pharmacological approach for treating dengue virus infection. The strategy of developing cysteine-directed covalent inhibitors of host factors may provide a new and general paradigm for developing host-directed agents with the potential to broadly protect against many infectious agents.